The present invention relates generally to a substantially non-aqueous heat transfer fluid for use in a heat exchange system and, particularly, to a coolant for internal combustion engines.
The coolants that are currently used create continuing environmental problems and raise concerns about toxicity, health effects and disposal problems. In particular, toxicity leading to acute short term oral health effects of coolants upon humans and other mammals is problematic. In addition, chronic health problems associated with coolants often relate to contamination from elemental heavy metal precipitates and toxic inhibitors that are added for water related reactions.
Every year nearly 700 million gallons of antifreeze are sold in the U.S. alone, and about 1.2 billion gallons are sold worldwide. The problem of the inherent toxicity of currently used coolants is exacerbated by estimates that 25% to 50% of this volume is disposed of improperly. One major cause of this pollution is dumping by consumers. While increased consumer awareness can be achieved through education, improper disposal will remain a problem.
Another major source of improper disposal emanates from leakage, spills and overflows in the heavy duty truck and off-road vehicle industry. Experience with heavy duty vehicles shows that it is common to lose 10% of the coolant volume every 12,000 miles (19,312 km) to 18,000 miles (28,967 km). This equates to a leakage rate of one drop per minute, or one gallon per month for the typical highway truck. Even though a coolant leak this small is likely to go unnoticed, it can accumulate into a significant loss. For example, many heavy duty fleets never change coolant but purchase enough antifreeze for loss replacement every year to replace all of the coolant in each of their vehicles.
In some heavy-duty operations, overflows and venting losses account for far more coolant loss than the previously mentioned leaks at the water pump, hose clamps or radiator core. When a heavy-duty truck radiator without an overflow tank is topped off, a quart or more of coolant is usually lost due to overflow from the coolant expanding upon heating of the engine. It is to be noted that even if small spills and leaks of coolant eventually biodegrade with little impact upon the environment, such leaks present a toxic danger to wildlife while they exist as a liquid and by contamination of heavy metals they carry (suspended), due to cooling system erosion and corrosion.
Current formulations of engine coolants typically utilize the characteristics of water as the primary heat removal fluid. The water content of a coolant is typically 30% to 70% by weight, depending upon the severity of the winter climate.
Another component of a conventional engine coolant is a freeze point depressant. Currently, the freeze point depressant in most cases is ethylene glycol (EG), which is used in a range of 30% to 70% by volume to prevent freezing of the water during winter. In some warm weather areas, freezing temperatures are not encountered and water with only a corrosion inhibitor package is used.
Moreover, an additive package containing numerous different chemicals is initially added to the freeze point depressant to form an antifreeze concentrate, and eventually blended with water to form the coolant. These additives are designed to prevent corrosion, cavitation, deposit formation and foaming, and are each in concentrations of 0.05% to 3% by weight of the final coolant.
Recently propylene glycol (PG), with inhibitors, has gained some acceptance as a freeze point depressant, mostly because of it's lower toxicity rating than EG. It is the water fraction and the delicate balance of the water content to the freezepoint depressant that has thwarted attempts by vehicle manufacturers and coolant formulators to develop a “world coolant” which is applicable from the artic to the tropics and to all engines; light and heavy duty. Currently a ratio of 70% EG concentrate to 30% water is required for the artic, but this ratio is not acceptable (due to convective transfer loss) in the tropics which typically require a 40% EG to 60% water ratio. Additionally the heavy duty engine manufacturers require a high concentration of sodium nitrate, as an additive for iron cavitation (see below), which is not desirable in light duty engines. The complexity of balancing various water to EG (or PG) ratios and different additive formulations has created a recurring problem in the field of improper freeze protection and clogged radiators and heater cores, due to mis-formulation of inhibitor additives. These problems, as will be discussed further below, exist because of the need for a substantial water fraction in the make-up of the cooling fluid, termed an “aqueous coolant”.
In addition, contaminants build up in the coolant as the engine is used and result from thermal and oxidative breakdown of glycol, lube oil and fuel accumulation, or metal corrosion and erosion products from the cooling system components.
Finally, as mentioned above, supplemental coolant additives are used in heavy duty service to prevent cavitation erosion of cylinder liners (iron) and to replenish inhibitor chemicals depleted with service. Supplemental coolant additives are not used or required in passenger cars which have a coolant life of 20,000 miles (32,186 km) to 30,000 miles (48,279 km). Heavy-duty service usually demands 200,000 miles (321,860 km) to 300,000 miles (482,790 km) before coolant replacement and hence the need to periodically replenish inhibitors. Examples of commonly used supplemental coolant additives include sodium nitrate, dipotassium phosphate, sodium molybdate dihydrate, and phosphoric acid.
Cylinder liner cavitation is another prime example of the complex reactions which occur when a substantial portion of the coolant is made up of water. When, for example, a mixture of 50% water and 50% EG is used (50/50 EGW) in a heavy duty engine the vapor pressure of the coolant is very high, about 600 mm/Hg, and under high load conditions large amounts of water vapor are produced on the coolant side of the cylinder wall. As the water vapor ultimately collapses around the cylinder wall, the energy released from the phase change (gas to liquid) impacts the wall and small amounts of iron are eroded, on an ongoing basis. Sodium nitrate is added to chemically limit the amount of vapor impacting the cylinder wall.
Supplemental coolant additives must be chemically balanced with the coolant volume, which is costly to control and can be catastrophic to the cooling system components, and the engine, if improperly done. If the amount of the supplemental coolant additives in the coolant is too low, corrosion and cavitation damage to the engine and cooling system components will occur, but if the amount is too high, additives will “fall-out” of solution and eventually clog radiator and heater cores. Another difficulty with supplemental coolant additives is that they are difficult to properly dissolve in an aqueous solution and may resist going into a final solution, as a supplemental additive, which causes additional clogging problems.
The acute oral toxicity of spent antifreeze is largely determined by the amount of ethylene glycol used. Thus, additives and contaminants have a lesser effect on coolant toxicity. Regardless of size, spills and leaks can pose an acute oral toxicity danger to wildlife and pets.
Glycols make up 95% by weight of the antifreeze/coolant concentrate, and after blending with the water, about 30% to 70% by volume of the coolant used in the vehicle. Conventional antifreeze has for years been formulated with EG.
A major disadvantage of using EG as a freeze point depressant for engine coolants is its high toxicity to humans and other mammals if ingested. Toxicity is generally measured in accordance with a rating system known as the LD50 rating system, which is the amount of substance expressed in grams per kilogram of body weight, when fed to laboratory rats in a single dose, which will cause an acute oral toxic poisoning. A lower LD50 value indicates a higher toxicity (smaller amounts of substance required to be lethal). An LD50 rating of less than or equal to 20.0 grams of substance per kilogram of body weight can classify a material as hazardous. Thus, because EG has an LD50 rating of 6.1 g/kg, EG is hazardous by this rating system. Moreover, EG is a known toxin to humans at relatively low levels, reported as low as 0.398 g/kg. Consequently, EG is classified by many regulatory authorities as a dangerous material. When ingested, EG is metabolized to glycolic and oxalic acids, causing an acid-base disturbance which may result in kidney damage. EG also has the added complication of a sweet smell and taste thereby creating an attraction for animals and children.
In addition to the difficulties that arise from the use of EG, serious problems may result from the instability of the additives that are used in current coolant formulations. An EG-based concentrate requires 3% to 5% water content in order not to freeze at +7.7° F. (−13.5° C.). Water is also added to all known coolant concentrates so that additives can be dissolved during formulation and remain in suspension during extended periods of storage.
Although a small amount of water, as discussed above, is intentionally added to EG/PG concentrate to keep the water soluble additives in solution, while being stored, it is not adequate for long periods of time. The additives must be agitated, periodically, in order to remain in solution until added to water as a final coolant mixture. If storage as a concentrate is too long a period (over 6-8 months) then the water soluble additives begin to “fall-out” of suspension and will accumulate in the bottom of the container as a “gel”. The “gelled” additives will not return to solution even with agitation. This problem, however, is not limited to the stored concentrate only. Even when fully mixed, as 50/50 EG or PG, the water soluble additives will “gel-out” if not agitated (the engine run) regularly. This can be a severe problem for engines used in stationary emergency pumps and generators as well as military and other limited use engines.
One difficulty with the large water fraction of the diluted engine coolant, typically a 50/50 ratio of concentrate to water, is the emergence of precipitates of heavy metals, such as lead and copper contaminants, that become suspended in the water portion of the circulating coolant in the engine. The water reacts with lead and copper materials from radiators which are the source of not only brass, and thereby copper, but also lead solder.
Water is also highly reactive with light alloys, such as aluminum, and the water fraction of the coolant can generate large amounts of aluminum precipitates, which increases at an increasing proportion with higher coolant temperatures. Water soluble additives are used for these reactions, but cannot totally eliminate the reaction, and aluminum is constantly lost to the 50/50 EG or PG coolant.
Cooling systems contain many different metals and alloys, and corrosion of these metals by coolants has been unavoidable because of the inclusion of water with the diol-based antifreezes, such as ethylene glycol or propylene glycol. Corrosion occurs because of the formation of organic acids in the coolant, such as pyruvic acid, lactic acid, formic acid, and acetic acid. The organic diols produce acidic oxidation products when in the presence of hot metal surfaces, oxygen from either entrapped air or water, vigorous aeration, and metal ions, each of which catalyze the oxidation process. Moreover, formation of lactic acid and acetic acid is accelerated in coolant solutions at 200° F. (93.3° C.) or above while in the presence of copper. Formation of acetic acid is further accelerated in the presence of aluminum in coolant solutions at 200° F. (93.3° C.) or above.
Among the metals and alloys found in cooling systems, iron and steel are the most reactive in the formation of acids, whereas light metals and alloys, such as aluminum, are considerably less reactive. As the oxidation of diols progresses, the level of organic acids formed with the water fraction rises and the pH of the coolant decreases, and therefore the corrosion of the metal surfaces increases.
Currently known and utilized coolants include buffers to counteract these organic acids. The buffers act to create a coolant with a higher initial pH of approximately 10 or 11. Thus, as the oxidation occurs, the pH decreases accordingly. Some examples of typically utilized buffers include sodium tetraborate, sodium tetraborate decahydrate, sodium benzoate, phosphoric acid and sodium mercaptobenzothiazole.
Buffers, in turn, also require water in order to enter into and remain in solution. As the buffer portion of the solution becomes depleted over time, the water fraction of the coolant reacts with the heat, air and metals of the engine, and as a result, the pH decreases because of the acids that form. Thus, corrosion remains a large problem in coolants that utilize water.
In fact, all known coolant formulations require the addition of water to solubilize additives used as buffers and anti-foam agents and for prevention of aluminum corrosion. Examples of such additives include phosphates, borates, silicates or phosphoric acids. In addition, these water soluble additives require heat, extreme agitation, and extensive time for the water to react and cause the additives to dissolve.
These requirements add significant cost and complexity to the formulation and packaging of antifreeze concentrates. The energy costs and time required for blending, before packaging, are a major factor in the processing costs. Also, the constant requirement to monitor the formulating process to assure a “proper blend” has become extremely costly as many of todays' additives, for aqueous concentrates, interfere with each other and can cause an incomplete solution and failure of the formulation process.
All currently used and previously known coolants require inhibitors to control the corrosive effects from the required water content. The inhibitors must be balanced so that they do not react with each other because that would otherwise minimize their individual purposes. For instance, phosphates and borates would decrease the protection of silicates on aluminum. Moreover, the inhibitors must not be in excess concentration, which is usually done to extend the depletion time, because that causes damage to system components. For example, “fall-out” from solution causes plugging of radiators and heaters. In addition, silicates, silicones, borates and phosphates are abrasive and erode heat exchanger tubes and pump impellers. Nevertheless, the inhibitors must still exist in a concentration which is adequate to protect all metals.
Thus, the additive package that is included in known coolant formulations typically consists of from 5 to 15 different chemicals. These additives are broken down into major and minor categories, depending upon the amount used in an engine coolant formulation:
MAJOR (0.05% to 3%)MINOR (<0.05%)BufferDefoamerCorrosion inhibitorsDyeCavitation inhibitorsScale inhibitorSurfactantChelates
In addition, some of the additives themselves are considered toxic, such as borates, phosphates, and nitrates. Thus, not only do all known coolant formulations include additives that require heat, extreme agitation and extensive time for the water to react and cause the additives to dissolve, but the additives themselves are sometimes toxic. Further, the additives require complex balancing which accommodates the prevention of interference between the additives, while also preventing the excessive presence of any one additive in the coolant.